Conventionally, such a lead screw used to be made by machining a screw thread in a metallic rod made of such materials as bronze and stainless steel. A lead screw made of such material has a high dimensional precision and is durable, but is relatively expensive to manufacture because of the various machining steps involved. Furthermore, there is a substantial difficulty in improving the efficiency of fabricating such lead screws, and the cost of each lead screw is therefore unacceptably expensive. In some applications, generation of metal powder is desired to be avoided, and the use of synthetic resin material may be preferred.
There have been attempts to produce lead screws by molding synthetic resin material, but such attempts have encountered the following problems.
Synthetic resin material can be readily molded into a desired shape, but tends to involve shrinking and cracking as well known in the art, and may warp and otherwise deform in time depending on the environment. Therefore, synthetic resin material cannot provide sufficient reliability and precision that are required in lead screws for converting a rotational movement into a linear movement.
In view of such problems, it has been proposed to mold a desired screw thread on the outer wall of a metallic tube with synthetic resin material, but because the attachment between the screw thread and the metallic tube cannot be made strong enough to entirely prevent the separation between the tube and the screw thread as required for the lead screw to perform its function, this proposal has never achieved any commercial success.
The inventor has therefore previously proposed a method for molding a lead screw comprising the steps of forming a plurality of holes in a wall of tube made of metal or the like at portions matched in position with a thread crest of a prescribed screw thread at a desired interval, preparing a metallic die assembly having a cavity provided with a thread groove corresponding to an array of holes provided in the metallic tube, placing the metallic tube in the cavity of the metallic die assembly with the array of holes of the metallic tube matched in position with the thread groove, and injecting resin material axially into the metallic tube (Japanese patent laid open (kokai) publication No. 2-113910). The synthetic resin material is filled into the tube and emerges from the holes to be filled into the thread groove of the cavity. Thus, the screw thread formed on the tube is connected to the portion of the material filled in the tube via the holes provided in the wall of the tube, and the thread can be securely attached to the tube.
However, it was discovered that the method for making a lead screw proposed in this Japanese patent publication involves certain difficulties in the design of the injection gate for synthetic resin material. More specifically, the synthetic resin must be injected into the tube with a sufficient pressure to allow the synthetic resin to flow through the holes provided in the tube, and the time period required for each molding process is therefore comparatively long. The molding process may be speeded up by increasing the number of the holes and/or the diameter of each hole, but it will reduce the mechanical strength of the tube.
Because such a lead screw is required to be connected to an electric motor, it is desirable to have the lead screw to be integrally incorporated with a motor rotor. However, when an attempt is made to integrally mold a rotor with a lead screw, because the rotor is typically made of an annular permanent magnet piece made of ferrite and other brittle material, the injected synthetic resin material may cause such a large internal pressure on the rotor that it may crack or is otherwise damaged under the pressure.